1. Field of the Invention
Aspects of the present invention relate to a flat panel display that uses a signal transmission method that transmits a differential signal, and more particularly to a flat panel display that includes a differential signaling system for matching impedance in the signal transmission method.
2. Description of the Related Art
In general, a cathode ray tube (CRT) is one of display devices which have been in wide use as a monitor for a television, a measuring instrument, or an information terminal. However, since the CRT is heavy and large, it is not suitable to miniaturization and light-weight requirements of smaller electronic devices.
Accordingly, in order to replace the CRT, various flat panel displays, such as a liquid crystal display (LCD), a plasma display panel (PDP), a field emission display (FED), and an organic light emitting display (OLED) have been studied and developed, which have advantages in light of miniaturization, lighter weight, and low electric power consumption requirements. The above described flat panel displays include various components and wirings for transmitting signals between the components.
Recently, aided by the development in electronic circuits and manufacturing process technologies, signals can be transmitted through the wirings at high speeds. To meet the high speed signal transmission requirements, a drive speed of the components has also become high.
Accordingly, various methods for transmitting the high speed signals between the components through the wirings have been adopted. For example, a signal transmission method such a low voltage differential signaling (LVDS) method or a reduced swing differential signaling (RSDS) method for transmitting a differential signal has been used.
A differential signaling system transmits a signal having different modes but having a same amplitude and a different polarity through a differential transmission line. Accordingly, the differential signaling system tends to remove a concentrated magnetic field and tends to couple an electric field. Accordingly, a high speed signal can be stably transmitted without a signal reflection, a skew (phase delay), or electro magnetic interference (EMI) due to the coupled electric field.
A typical flat panel display will be described with reference the accompanying drawings in detail.
FIG. 1 is a block diagram showing a composition of a flat panel display. With reference to FIG. 1, the flat panel display includes a display panel 40, a gate driver 20, a data driver 30, and a controller 10. Pixels (not shown) are arranged at the display panel 40 in a matrix pattern. The gate driver 20 sequentially applies a scan signal to gate wirings of the display panel 40. The data driver 30 applies an image signal DATA1 to data wirings of the display panel 40. The controller 10 applies the image signal DATA1 from an external graphic controller (not shown) to the data driver 30, and applies a control signal CS1 to the gate driver 20 and the data driver 30 in order to control a drive timing. In the flat panel display, after all gate wirings of the display panel 40 are sequentially scanned and the image signal DATA1 is applied to the pixels through the data wirings to display one frame of an image, a vertical synchronous signal VSYNC is applied to display a next frame of the image.
FIG. 2 is a block diagram showing a controller 110 and a data driver 130 in detail. FIG. 3 is a view showing a signal transmission method between the controller 110 and the data driver 130. With reference to FIG. 2, the data driver 130 comprises a plurality of data driving circuits 132. The plurality of data driving circuits 132 receive image signals DATA [+,−] from the controller 110 through first and second wirings W1 and W2, and receive a control signal CS11 from the controller 110 through a third wiring W3.
The data driving circuits 132 receive image signals DATA [+,−] from the controller 110, and output the image signals DATA [+,−] to the data wirings according to the control signal CS11 from the controller 110. Although not shown in the drawings, a plurality of data wirings are electrically coupled to the data driving circuits 132, and applies the image signals DATA [+,−] that are applied to the data driving circuits 132 and to the pixels. Here, the image signals DATA [+,−] from the controller 110 are transmitted to the respective data driving circuits using the aforementioned differential signal transmission method.
FIG. 3 shows a signal transmission method between the controller 110 and the data driver 130 using a representative diagram of the controller 110, the data driver 130, and a connection thereof. As shown in FIG. 3, in order to transmit data (as image signals DATA [+,−]), an arrangement of differential transmission lines, namely, first and second wirings W1 and W2, is provided between the controller 110 being a sending end Tx and the data driving circuit 132 being a receiving end Rx. A termination resistor Rt is provided between the differential transmission lines at the receiving end (data driving circuit 132) side. The termination resistor Rt electrically connects the first wiring W1 and the second wiring W2 to each other, and the first wiring W1 and the second wiring W2 are coupled to each data driving circuit 132.
Accordingly, the image signal DATA [+] applied through the first wiring W1 is transferred back to the controller 110 through the termination resistor Rt and the second wiring W2. The termination resistor Rt prevents an excessive current from flowing in the data driving circuit 132, and a voltage across the termination resistor Rt is the image signals DATA [+,−], which are applied to the data driving circuit 132.
A plurality of electric components and wirings are provided in the flat panel display, which are electrically coupled to each other. Since the electric components and wirings have impedance values, a signal is attenuated during transmission of the signal between the electric components. That is, the controller 110 and the data driving circuits 132 have impedance values. Further, the first and second wirings W1 and W2 for connecting the controller 110 and the data driving circuits 132 have an impedance value of Z0.
If the impedance value Z0 of the first wirings W1 and W2 is different from that of the data driving circuits 132, namely, when an impedance mismatch occurs, the image signals DATA [+,−] are not accurately supplied to the data driving circuits 132. That is, a part of the image signals DATA [+,−] is reflected and discharged.
In detail, a reflection coefficient Γ is expressed by a following equation 1.
                    Γ        =                                            Z              diff                        -                          R              t                                                          Z              diff                        +                          R              t                                                          [                  Equation          ⁢                                          ⁢          1                ]            
where, a differential impedance Zdiff is a value that is less than a sum (2Z0) of impedance values of the first and second wirings W1,W2, and has a different value according to variations in a manufacturing process and a composition of the flat panel display.
Namely, when the differential impedance Zdiff is identical with a value of the termination resistor Rt, a reflection loss of the signals does not occur due to the matched impedances. However, the differential impedance Zdiff varies in practice. Accordingly, in the typical case, the impedance matching (or matched impedance) is not normally achieved when using the differential signal transmission method. When a reflection wave occurs due to mismatched impedances, an interference with the image signals DATA [+,−] applied through the first wiring W1 occurs to cause an unstable wave, and distortion and attenuation of the image signals DATA [+,−]. Also, the electro magnetic interference (EMI) deteriorates an image quality of the flat panel display.
Accordingly, in the differential signaling method, whether the impedance matching is achieved or whether a minute variation of differential impedance Zdiff occurs should always be monitored. However, since a typical method for detecting the minute variation in the differential impedance Zdiff has a long measuring time and uses measuring equipment of high cost, its disadvantages include increased testing cost and low detection rate for a minute variation in the differential impedance Zdiff.